The present disclosure relates to a gallium nitride (GaN) based semiconductor light-emitting device and a method for producing the same.
In light-emitting devices (GaN based semiconductor light-emitting devices) including an active layer composed of gallium nitride compound semiconductor, control of the band gap energy by means of the (alloy) composition and the thickness of the active layer results in a wide emission wavelength range from ultraviolet to infrared. GaN based semiconductor light-emitting devices, such as light-emitting diodes (LEDs), that emit light rays of various colors are already commercially available and used for wide applications, such as image displays, luminaries, inspection apparatuses, and sterilizing light sources. Furthermore, blue-violet semiconductor lasers have also been developed and used as pickups for reading and writing of large-capacity optical discs.
A typical GaN based semiconductor light-emitting device has a structure of a first GaN based compound semiconductor layer with an n-type conductivity, an active layer, and a second GaN based compound semiconductor layer with a p-type conductivity stacked in that order on a substrate.
In such a GaN based semiconductor light-emitting device, U.S. Pat. No. 6,635,904 B2 discloses a technique in which a smoothing layer is formed by crystal growth between an active layer and a first GaN based compound semiconductor layer. The formation of the smoothing layer significantly improves characteristics. A similar report discloses that the formation of an InGaN layer having an In content of about 0.04 and functioning as an underlying layer of an active layer that has a quantum well structure and that emits blue-violet light increases photoluminescence efficiency to 70%, which have been 30% to 40% in the past (see, T. Akasaka, H. Gotoh, T. Saito, and T. Makimoto, Appl. Phys. Lett., vol. 85, pp. 3089-3091 (2004)).
It has been known that a technique for the formation of a second GaN based compound semiconductor layer above an active layer having a superlattice subjected to uniform or modulation doping, the superlattice containing a Mg-doped AlGaN sublayer and a Mg-doped GaN sublayer. The formation of the second GaN based compound semiconductor layer having the superlattice increases hole concentration (for example, see K. Kumakura and N. Kobayashi, Jpn. J. Appl. Phys. vol. 38 (1999) pp. L1012; P. Kozodoy et al., Appl. Phys. Lett. 75, 2444 (1999); and P. Kozodoy et al., Appl. Phys. Lett. 74, 3681 (1999)). This technique is one in which high hole concentration is two-dimensionally obtained on the basis of the piezoelectric effect due to strain. Furthermore, it is possible to obtain the effect on conduction in the second GaN based compound semiconductor layer in the thickness direction, i.e., it is possible to reduce series resistance, by optimization of the superlattice period.
GaN based semiconductor light-emitting devices may be required to achieve high luminous efficiency in operation at a high operating current density (for example, an operating current density of 50 A/cm2 to 100 A/cm2 or more) and to achieve a significant reduction in operation voltage. However, none of the references describe specific means and measures for achieving such demands.
It is desirable to provide a GaN based semiconductor light-emitting device having a structure providing high luminous efficiency in operation at a high operation current density and a significant reduction in operation voltage. Furthermore, it is desirable to provide a method for producing the GaN based semiconductor light-emitting device.